There are two diffusion processes, molecular and turbulent, which should be modelled in different ways. Molecular diffusion is modelled by Wilke's formula for the multi-component gas diffusion, where the diffusion constants are dependent on the relative concentrations. Turbulent diffusion is basically modelled by the k- model with modifications for low Reynolds number flow effects. Numerical diffusion is eliminated by a corrective scheme which is based on accurate prediction of cross-flow diffusion. The corrective scheme in a fully explicit treatment is both conservative and stable, therefore can be used in long transient calculations. The corrective scheme allows relatively large mesh sizes without introducing the false diffusion and the time step size of the same order of magnitude as the Courant limit may be used.Several validation calculations are reported. Demonstration problems include the successful reproduction of analytical or known solutions, simulation of large scale experiments and analyses of "thought experiments" which test the physical reasonableness of the predictions. In particular, simulation of hydrogen transport tests performed at the Battelle Frankfurt Institute and Hanford Engineering Development Laboratory show good agreement with measured hydrogen concentration, temperature and flow fields. The results also indicate that potential areas of improvement are enhanced computational efficiency, further reduction of numerical diffusion and development of containment spray models. Overall, a useful tool applicable to many nuclear safety problems is described.